Ph value measuring device comprising in situ calibration means

ABSTRACT

The invention concerns a device for measuring the pH of an effluent, said device comprising means for measuring an item of information representative of the pH of said effluent intended to be brought into contact with said effluent. 
     According to the invention, such a device further comprises means for modifying the pH value of said effluent close to said means for measuring.

1. FIELD OF THE INVENTION

The field of the invention is that of techniques for measuring the valueof the pH of a liquid effluent.

More specifically, the invention pertains to the designing andmanufacture of probes and to a method for the continuous measurement ofthe value of the pH of a liquid effluent.

2. PRIOR ART

Potential hydrogen, more commonly called pH, represents the chemicalactivity of hydrogen ions in solution. The value of the pH of a solutionreveals its acidity or its basicity.

The pH is a parameter used in many applications.

The pH is for example used in water treatment where it is an indicator,for example of the healthy biological condition of water. It is alsooften used as a control parameter when implementing water treatmentmethods.

The pH is also often used in microbiology since its value governs enzymereactions and the growth of bacteria. The pH is also used in thepharmaceutical and medical fields since minute variations in pH can besymptomatic of serious metabolic disturbances.

There are numerous techniques for measuring the value of the pH of asolution. Among these there are especially:

pH paper whose color varies when it is put into contact with a solutionaccording to the pH value of this solution;

glass electrode probes;

non-glass electrode probes.

Only glass electrode probes or non-glass electrode probes are suited tocarrying out a continuous measurement of the pH of a solution.

Glass electrode probes are relatively brittle and require daily orweekly maintenance operations, especially because the glass electrodecontains an electrolyte, which is a consumable. This drawback can bereduced through the use of electrolyte in the form of gel but cannot becompletely removed. Besides, the storage of glass electrodes impliescompliance with special and constraining conditions. Indeed, glasselectrodes have to be stored in a potassium chloride solution since drystorage induces premature ageing.

Non-glass electrode probes have especially been developed in order toovercome these drawbacks.

The invention relates more particularly but not exclusively to non-glasselectrode pH measuring probes.

As can be seen in FIG. 1, non-glass electrode pH measuring probesclassically comprise an ISFET (Ion-Sensitive Field Effect Transistor)type transistor and a reference electrode 15.

The ISFET transistor comprises a substrate 10 generally made of siliconon which are placed a doped source 11, a doped drain 12 and a gate 13separated from the source 11 and the drain 12 by an insulator 14.

The gate 13 has a layer sensitive to variations in H⁺ ion concentration.

In some variants, the reference electrode 15 can be constituted by aMOSFET (Metal Oxide Semiconductor Field Effect Transistor) typetransistor.

The layer that is sensitive to variations in the H⁺ ion concentration aswell as the reference electrode 15 are to be placed in contact with thesolution E, the pH of which is to be measured.

The source 11 and the drain 12 are connected to a generator 16 ofelectric voltage and electric current capable of generating a voltageand an electric current of constant values at their terminals.

The reference electrode 15 and the source 11 are connected to a means,such as a voltmeter, for measuring a so-called control voltage 17. Thisvoltmeter is capable of measuring a voltage at their terminals. Inasmuchas the reference electrode is connected to the contact of the gate, thevoltage measured by the voltmeter is a voltage V_(GS) of the ISFETtransistor across the gate and the source.

In order to measure the pH of a solution, it is put into contact withthe gate 13 and the reference electrode 15.

The current generator and the voltage generator 16 are used to generatethe passage of a constant current and a constant electric voltagebetween the source 11 and the drain 12. The values of this voltage andof this current are stable and high enough to enable the transistor tobe biased.

The variation in the pH of the solution to be analyzed induces thevariation of its electrochemical potential which modifies the voltageV_(GS) of the transistor. The gate-source voltage V_(GS) varies linearlyaccording to the pH for a drain-source current I_(DS) and a drain-sourcevoltage V_(DS) that are constant. The voltage V_(GS), called a controlvoltage, is then measured at the terminals of the reference electrode 15and the source 11. The measurement of this voltage therefore enables thevalue of the pH of the solution to be determined

As compared with glass electrode probes, ISFET electrode probes are moreresistant, easier to store since they can be stored in a dry state, moreprecise and faster because they have a very short response time.

ISFET electrode probes and more generally probes for measuring pH canhowever be further improved.

3. DRAWBACKS OF THE PRIOR ART

The main drawback of ISFET electrode probes is related to the fact thata drift is observed over time between the measured value of the pH andits real value. This drift dictates the regular recalibration of theprobe.

In order to ensure that the measurement of the pH by the proberepresents reality, the frequency of the recalibration is generallydaily.

The linear function relating the pH to the voltage V_(GS) measured bythe ISFET probe is:

V _(GS) =C ₂·pH+E ⁰

where C₂ (the slope) and E⁰ (the intercept point) are constants.

In the recalibration phases, the probe is dismounted so as to be placedalternately in solutions having pH values that are known and differentfrom one another. The comparison of the pH values measured by the probewith the real values then allows to correct the value of the slopeand/or the intercept point of the pH curve of the probe in such a waythat the pH value measured with the probe is identical to the real valueof the pH of the solution analyzed.

These recalibrations therefore require qualified workers, which entailsa cost factor that can be high.

They require the dismounting of the probe, which can be a lengthy andirksome task since the probe is not always very accessible.

In addition, the recalibration phases dictate the stoppage of theprocesses in which the pH is used as a control parameter. This leads toa loss of productivity. Thus, in certain water treatment methods, therecalibrations induce a drop in the production of treated water.

4. GOALS OF THE INVENTION

The invention is aimed especially at overcoming the drawbacks of theprior art.

More specifically, it is a goal of the invention to provide a techniquefor measuring the pH, which enables the reduction, in at least oneembodiment, of the frequency of the recalibrations as compared with thetechniques of the prior art.

It is another goal of the invention to implement a technique of thiskind that makes it possible, in at least one embodiment, to simplify therecalibration operations.

In particular, it is a goal of the invention, in at least oneembodiment, to procure a technique of this kind that does not requirethe dismounting of the probe to carry out its recalibration.

It is another goal of the invention, in at least one embodiment, toprocure a technique of this kind that does not call for the probe to beput into contact with various solutions having different known pH valuesin order to carry out its recalibration.

It is yet another goal of the invention to provide a technique of thiskind which, in at least one embodiment, is simple to implement and/or tostore and is reliable and/or robust and/or precise.

It is another goal of the invention, in at least one embodiment, toprocure a technique of this kind which does not necessitate the use ofreagent such as a liquid electrolyte or reagent in the form of gel tocarry out the measurement because this gel would have to be renewedduring maintenance, as is the case with glass electrodes where thereference electrode bathes in an electrolyte.

5. SUMMARY OF THE INVENTION

These goals as well as others that shall appear here below are achievedby means of device for measuring the pH of an effluent, said devicecomprising means for measuring a piece of information representing thepH of said effluent, that are to be put into contact with said effluent.

According to the invention, such a device also comprises means formodifying the value of the pH of said effluent in proximity to saidmeans for measuring.

Also according to the invention, such a device in addition preferablycomprises means for calibrating said device for measuring, said meansfor calibrating being configured to calibrate the device for measuringafter modification of said pH of said effluent in proximity to saidmeans for measuring by said means for modifying.

Thus, the invention relies on a wholly original approach in which meansare integrated, into a pH measurement probe, to modify the pH of theeffluent locally, i.e. in proximity to the active part of the probe(elements of the probe in contact with the effluent at the level atwhich the measurement is made).

It is thus possible to locally modify the value of the pH of theeffluent so as to carry out the calibration of the probe withoutdismounting it or plunging it into different buffer solutions of knownpH values in order to calibrate the device.

The technique according to the invention thus makes it possible to carryout an in situ calibration without dismounting the pH probe andtherefore facilitates the calibration, reduces the time needed forcalibration and reduces its inherent cost.

According to a preferred embodiment, said means for measuring comprise:

an ISFET type transistor comprising a source and a drain disposed on asubstrate, and a gate that is to be put into contact with said effluent;

a reference electrode;

means for generating a constant voltage at the terminals of said sourceand said drain;

second means for generating a constant electric current between saidsource and said drain;

means for measuring a control voltage V_(GS) at the terminals of saidsource and said reference electrode,

means for determining the value of the pH of said effluent as a functionof the value of said control voltage, the value of the control voltageV_(GS) and the value of the pH being preferably related by a formula ofthe type:

V _(GS) =C ₂·pH+E ⁰, where E⁰ and C₂ are predefined constants;

said means for calibrating being preferably configured to implementphases or steps of calibration during which:

they act on said means for modifying the value of the pH to take itmomentarily to at least one first known value pH₁, then

they act on said means for measuring a control voltage V_(GS), tomeasure its corresponding value V_(GS1);

they compute the value of the constant E⁰ according to the values of pH₁and of V_(GS1).

Thus, the invention relies in this embodiment on a wholly originalapproach in which there is integrated, into a probe for measuring the pHof a type comprising an ISFET transistor, means to locally modify the pHof the effluent and means to calibrate, in situ, the device formeasuring.

To measure the pH of a solution, the reference electrode and the gateare put into contact with it.

When the probe is put into contact with the effluent of which it wishesto measure the pH, the H⁺ ions that it contains modify theelectrochemical potential of the solution and therefore the voltageV_(GS) of the ISFET.

Means for generating a constant electric current and a constant voltageare then implemented to generate a constant current and a constantvoltage at the terminals of the source and the drain, the values ofwhich are chosen to enable the ISFET transistor to be biased.

The control voltage V_(GS) is then measured at the terminals of thesource and the gate or more specifically of the reference electrode. Thevalue of this control voltage varies according to the pH of theeffluent. The pH of the effluent is then determined according to thevalue of the control voltage.

So that the probe does not perceive a drift between the value of pHmeasured by means of the probe and the real value of the pH, this probeis regularly calibrated in situ. To this end, the means for calibratingpreferably act on the means for modifying of the pH to locally carry thevalue of the pH of the effluent to a known value. They then command themeasurement of V_(GS) and then compute the value of E⁰ which is theintercept point of the curve of the voltage V_(GS) as a function of thepH.

Said means for modifying the value of the pH preferentially comprise ananode and a cathode to be put into contact with said effluent, and firstmeans for generating electric current between said anode and saidcathode.

The invention in this case relies on a wholly original approach whichconsists of the integration, into a pH-measuring probe of the typecomprising an ISFET transistor, of an anode and a cathode planned tocome into contact with the effluent to be analyzed, and means forgenerating an electric current at the terminals of this electrodes.

So that the probe does not perceive a drift between the value of pHmeasured by means of the probe and the real value of the pH, this probeis regularly calibrated in situ. To this end, an electric current isapplied between the anode and the cathode. Thus, the production ofprotons is generated in the effluent in proximity to the measurementmeans; in the case of an ISFET, at the proximity of the active surfaceof the gate, by oxidation of water according to the formulaH₂O→2O₂+4H³⁰4e⁻. The pH of the effluent can thus be modified locally ina controlled manner.

Indeed, by regulating the value of the electric current between theanode and the cathode, it is possible to control the value of the pH inproximity to the active part of the measuring means, in the case of anISFET in proximity to the active surface of the gate. It is thuspossible successively to place the pH at one or more different knownvalues in order to calibrate the probe.

The technique according to the invention therefore makes it possible tocarry out the calibration of the probe, also called a recalibration, insitu, i.e. without dismounting it and without influencing the medium inwhich the measurement is made. Indeed, the quantity of H⁺ ions generatedis small as compared with the volume of liquid in which the measurementis made.

The technique of the invention therefore takes part in facilitating thecalibration of a pH measuring probe of the type comprising an ISFETtransistor and accordingly reducing the cost inherent in thiscalibration.

Said device, for example said means for modifying the pH, preferablycomprises command means to implement or not implement said first meansfor generating an electric current.

In this case, said means for calibrating are preferably configured toact on said command means to implement said first means for generatingan electric current to modify the value of the pH during said phases ofcalibration.

Thus, it is possible that the first means of current generation will notbe implemented to carry out a classic measurement of pH and then couldbe implemented to locally modify the pH before carrying out newmeasurements of pH in order to calibrate the device.

According to a preferred characteristic, a device according to theinvention comprises a membrane permeable to the H⁺ ions covering andbeing in contact with said anode and at least partly said means formeasuring, in particular the active part of these means.

When the probe is put into contact with the effluent for which the pH isto be measured, the H⁺ ions that it contains spread within the membranein order to reach an equilibrium of concentration between the interiorand the exterior of the membrane. The concentration in H⁺ ions insidethe membrane is then identical to that of the effluent. The measurementof the pH is therefore done within the membrane. During the calibration,the pH is modified only within the membrane, i.e. in a restrictedvolume. In particular, the precision of calibration is improved.

When the device for measuring is of the type comprising an ISFET typetransistor, and when it comprises a membrane permeable to H⁺ions, thismembrane covers said gate and said anode, said gate and said anode beingin contact with said membrane.

When the probe is put into contact with the effluent for which the pH isto be measured, the H⁺ ions that it contains spread within the membranein order to reach an equilibrium of concentration between the interiorand the exterior of the membrane. The concentration in H⁺ ions insidethe membrane is then identical to that of the effluent. A constantelectric current and a constant electric voltage of sufficient value areapplied between the source and the drain in order to bias thetransistor. A control voltage V_(GS) is then measured at the terminalsof the reference electrode and the source, the value of which varies asa function of the pH of the effluent. The pH of the effluent is thendetermined according to the value of the control voltage.

During the calibration, an electric current is applied between the anodeand the cathode. Thus, the process generates the production of protonsinside the membrane by oxidation of water according to the formulaH₂O→2O₂+4H⁺4e⁻. The pH of the effluent can thus be modified locally,easily and rapidly in the membrane without interfering with the externalmedium. The membrane plays the role of a buffer between the externalmedium and the sensor, the electrodes placed beneath this membraneenabling the pH around the sensor to be modified at will.

Implementing the membrane makes it possible to vary the value of the pHonly in the membrane, i.e. in a restricted volume. The value of thevoltage between the anode and the cathode generates a constantproduction of protons within the membrane that is more stable than whenthe membrane is not used. The reliability of the device and theprecision of the calibration are thus improved.

Said membrane is preferably made of polymer such as, for example,poly(2-hydroxyethylmethacrylate).

The use of a polymer and especially of poly(2-hydroxyethylmethacrylate)gives a membrane permeable to the H⁺ ions, the use of which givesefficient results in terms of control of local variation of pH.

A device according to the invention preferably comprises means tocalibrate said device from at least one measurement of said controlvoltage after an implementation of said first means for generating anelectric current by said command means.

In some variants, said reference electrode could include a MOSFETtransistor or any other reference pseudo-electrode such as for example asilver-silver chloride wire, gold wire, etc.

The MOSFET transistor fulfils the function of the reference electrode,and the measured control voltage which is proportional to the pH of thesolution analyzed, is the voltage at the terminals of the gates of theMOSFET and the ISFET. In this case, the MOSFET is completelyencapsulated. Only the gates of the ISFET as well as the anode and thecathode can be put into contact with the effluent to be analyzed.

In the case of the ISFET, the reference electrode will be designed to beput into contact with the effluent to carry out the measurement of thepH.

The invention also relates to a method for measuring the pH of aneffluent by means of a device according to any one of the variantsdescribed here above.

Such a method comprises:

a phase or step for measuring the pH with said means for measuring;

a step for calibrating comprising, in addition to the previous step, atleast one step for modifying the value of the pH of said effluent inproximity to said means for measuring with said means for modifying thevalue of the of said effluent.

More specifically, such a method preferably comprises:

a phase for measuring the pH with said means for measuring;

a step for calibrating comprising at least one step for modifying saidpH of said effluent in proximity to said means for measuring by saidmeans for modifying, and a step for calibrating said device formeasuring by said means for calibrating.

According to a first preferred embodiment, said step for measuring thepH comprises:

a step for generating a constant voltage at the terminals of said sourceet said drain;

a step for generating a constant electric current at the terminals ofsaid source et said drain;

a step for measuring control voltage V_(GS) at the terminals of saidsource and said reference electrode;

a step for determining the value of said pH as a function of the valueof the control voltage V_(GS), the value of the control voltage V_(GS)and the value of the pH being preferably related by the formula of thetype: V_(GS)=C₂·pH+E⁰, where E⁰ and C₂ are predefined constants.

Said step for calibrating preferably comprises at least:

a step for modifying said pH by said means for modifying the value ofthe pH to take it momentarily to a first known value pH₁, then

a step for measuring the corresponding control voltage V_(GS1) by saidmeans for measuring control voltage;

a step of computation by said means for calibrating of said constant E⁰as a function of the known values pH₁ and V_(GS1).

In this case, the calibration consists in modifying the value of theintercept point of the control voltage V_(GS) expressed as a function ofpH.

According to a second embodiment, said step for calibrating comprises

a step for modifying said pH by said means for modifying the value ofthe pH to take it momentarily to a first known value pH₁, then

a step for measuring the control voltage V_(GS1) by said means formeasuring a control voltage V_(GS), to measure the value V_(GS1)corresponding to the first value pH₁, then

a step for modifying said pH by said means for modifying the value ofthe pH to take it momentarily to a second known value pH₁,

a step for measuring the control voltage V_(GS2) by said means formeasuring a control voltage V_(GS) to measure the value V_(GS2)corresponding to the second value pH₂;

a step of computation by said means for calibrating of the value of saidconstants E⁰ , C₂ as a function of the known values of pH₁, pH₂, V_(GS1)and of V_(GS2).

In this case, the calibration consists in modifying the value of theintercept point E⁰ and the slope C₂ of the control voltage V_(GS)expressed as a function of the pH.

In variants of the invention, said step for calibrating, especially saidstep for modifying the pH, could include a step for generating aconstant electric current between said anode and said cathode.

Said step or steps for modifying said pH could include a step forimplementing said first means for generating an electric current.

In variants, said step for calibrating could be implemented at apredetermined frequency, preferably daily. Said step for measuring couldbe implemented continuously or not continuously. The measurement of thepH will naturally be stopped during the calibration except for the pHmeasurement needed for the calibration. The frequency of implementationof the calibration could be adjustable.

The invention also concerns an element for measuring the pH of a deviceaccording to any one of the variants explained here above. Such aelement comprises:

an ISFET type transistor comprising a source and a drain disposed on asubstrate, and a gate to be put into contact with said effluent;

a reference electrode;

means for connecting means for generating a constant voltage at theterminals of said source and said drain;

means for connecting second means for generating a constant electriccurrent at the terminals of said source and said drain;

means for connecting means for measuring a control voltage at theterminals of said source and said reference electrode;

an anode;

a cathode;

means for connecting a first means for generating an electric currentbetween said anode and said cathode.

6. LIST OF FIGURES

Other features and advantages of the invention shall appear more clearlyfrom the following description of a preferred embodiment, given by wayof a simple, illustratory and non-exhaustive example, and from theappended drawings, of which:

FIG. 1 illustrates a probe for measuring the pH according to the priorart;

FIG. 2 illustrates a probe for measuring the pH according to theinvention;

FIGS. 3 and 4 illustrate curves showing the evolution in time of thereal value of the pH of a solution, the value of the pH measured bymeans of a probe according to the prior art, and the value of the pHmeasured by means of a probe according to the invention;

FIG. 5 illustrates the diagram of a part of an electric circuit of aprobe according to the invention, the reference electrode of which isconstituted by a MOSFET transistor.

7. DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION 7.1. Reminder of theGeneral Principle of the Invention

The general principle of the invention consists of the integration, intoa pH measuring probe, of means for modifying the pH of the effluentlocally, i.e. in proximity to the active part of the probe (elements ofthe probe in contact with the effluent at which the measurement ismade).

It is thus possible to locally modify the value of the pH of theeffluent so as to carry out the calibration of the probe withoutdismounting it or plunging it into different buffer solutions havingknown pH values.

The technique according to the invention thus enables a calibration tobe done in situ without dismounting the pH probe and thereforefacilitates the calibration, reduces the time needed for the calibrationand reduces the cost inherent in this calibration.

In one variant, the invention consists of the integration, into a pHmeasuring probe of a type comprising an ISFET transistor, of an anodeand a cathode to be put into contact with the effluent to be analyzedand of means for generating an electric current at the terminals of theanode and the cathode.

To measure the pH of an effluent, the probe is put into contact with it,a constant voltage V_(DS) is applied to the terminals of the drain andthe source and a constant electric current I_(DS) is put intocirculation across these terminals in order to bias the transistor.Then, the generation of a voltage V_(GS) is observed at the terminals ofthe source and the gate, the value of which is proportional to that ofthe pH of the effluent. This voltage is measured and then the pH of theeffluent is determined as a function of the value of voltage measured.

In order to regularly calibrate the probe, an electric current iscreated between the anode and the cathode. Thus, the production ofprotons is generated in proximity to the active surface of the gate inorder to locally modify the pH of the effluent.

In regulating the value of the electric current across the anode and thecathode, it is possible to control the local value of the pH. It is thuspossible to successively place the pH locally at one or more differentknown values and thus carry out the calibration of the probe.

The technique of the invention therefore plays a part in facilitatingthe calibration of a pH measuring probe of a type comprising an ISFETtransistor and therefore in reducing its inherent cost.

7.2. Example of One Embodiment of a Probe According to the Invention7.2.1. Architecture

Referring to FIG. 2, we present an embodiment of a device for measuringpH according to the invention, also called a pH measuring probe.

As shown in this FIG. 2, such a probe comprises an ISFET typetransistor. This probe classically comprises a source 21 and a drain 22placed on a substrate 23. Classically, the drain and the source aredoped. Their doping could respectively be N type or P type doping, orvice versa, depending on the type of flow between the source and thedrain. It also classically comprises a gate 24. The gate 24 is separatedfrom the source 21 and the drain 23 by an insulator 25 and comprises asurface sensitive to the H⁺ ions. In this embodiment, the gate is madeout of Ta₂O₅.

The probe also comprises a reference electrode 26, which is connectedwith the gate contact of the electronic control circuitry and enablesthe measurement of the variations in potential at the contact of thegate 24. It also comprises an anode 27 and a cathode 28.

In this embodiment, the anode is made of platinum and the cathode ismade of stainless steel. Other suitable materials can also be used.

The anode 27 extends all around the gate 24 without being in contactwith it.

The anode 27 as well as the gate 24 are coated with a membrane 29 withwhich they are in contact. This membrane 29 is permeable to the H⁺ ions.It is made out of polymer such as for examplepoly(2-hydroxyethylmethacrylate), agarose, polyvinyl alcohol (PVA), etc.It preferably takes the form of a gel. Its thickness preferably rangesfrom 40 to 150 microns. It is preferably fixedly attached to the anodeand to the gate by covalent bonds.

This membrane 29 as well the reference electrode 26 and the cathode 28are designed to be put into contact with the effluent E, the pH of whichis to be measured.

The probe comprises means 30 for generating a voltage V_(DS), such as avoltage generator, and a current generator I_(DS), such as an electriccurrent generator, which are connected to the terminals of the source 21and the drain 22 through means for connecting provided for this purpose.These means enable the application of a voltage V_(DS) of constant valueand an electric current I_(DS) of constant value between the source andthe drain.

The probe comprises means for measuring a voltage V_(GS) between thegate 24 and the source 21 such as for example a voltmeter 31. In thisembodiment, these means for measuring are connected to the source and tothe reference electrode. They enable the measurement of V_(GS) since thegate and the reference electrodes are both in contact with the effluentto be analyzed.

This voltage varies as a function of the pH of the solution to beanalyzed.

The probe comprises means 32 for generating an electric current, such asan electric current generator, that are connected to the terminals ofthe anode 27 and the cathode 28 through means for connecting providedfor this purpose. These means for generating current enable thegeneration of a constant electric current between the anode and thecathode. This current enables the generation of a fixed concentration ofprotons proportional to the pH.

The probe comprises a command means for acting on the means 32 forgenerating current so as to control the intensity of the current thatthey deliver. The means for generating current can thus generateconstant currents of different values for predetermined durations.

The application of an electric current at the terminals of the cathodeand the anode by means of the generator 32 enables the generation of theproduction of H⁺ protons in the membrane by oxidation of water accordingto the formula:

H₂O=<2O₂+4H⁺+4e−

and the modifying of pH therein. The concentration in protons [H⁺]generated is proportional to the intensity i of the current imposedbetween the anode and the cathode:

[H⁺]=C₁·C

where C₁ is a constant and i is the intensity of the current imposedbetween the anode and the cathode.

In general, the value of the constant C₁ can be determined during afirst step and a second step of initial setting in the factory. Thefirst step consists in carrying out a calibration of the ISFETtransistor probe with solutions having known pH values, withoutgenerating any current between the anode and the cathode. In anotherstep, different values of current are applied between the anode and thecathode and the corresponding pH is measured by means of the ISFETtransistor probe. A characteristic curve linking the value of themeasured pH and the generated current is obtained by linear regression.It establishes the value C₁ necessary for the in situ calibration, C₁being its slope. The constant C₁ is thus determined during themanufacture of the probe.

The probe comprises means for determining the value of the pH of theeffluent according to the value of the voltage V_(GS) measured at theterminals of the gate 24 and the source 21.

The function linking the pH to the measured voltage V_(GS) is:

V _(GS) =C ₂·pH+E ⁰

where C₂ and E⁰ are constants to be determined

This formula corresponds to a generalization of the Nernst equation:

V _(GS)=(−2.3 RT/nF)·pH+E ⁰

With:

E⁰: constant

R: constant of gases

F: Faraday constant

T: temperature in degrees Kelvin

n: ion charge

The Nernst equation gives the theoretical values of the slope C₂ (of theorder of 59 mV per pH unit) and of E⁰ (intercept point which depends onthe threshold voltage of the transistor and can vary from one sensor toanother). However, E₀ and C₂ are capable of varying for each ISFETtransistor probe. These constants must therefore be determined preciselyduring the initial calibration at the manufacture of each probe withsolutions of known pH values.

In one embodiment, the initial calibration of the probe comprises afirst measurement of the voltage V_(GS), called V_(GS1), in a firstsolution at a first value of pH, pH₁, and then a second value of thevoltage V_(GS), called V_(GS2), in a second solution at a second valueof pH, pH₂. The values of the constants E₀ and C₂ can then be computedby applying the following formulae:

C ₂=(pH₁−pH₂)/(V _(GS1) −V _(GS2))

E ⁰ =V _(GS1)−(pH₁ −pH ₂)/(V _(GS1) −V _(GS2))·pH ₁

The command means and the means for determining the value of the pHcomprise a microcontroller.

The probe comprises means for calibrating. These means for calibratingcomprise the microcontroller which enables the pH measurement cycles andprobe calibration cycles to be carried out in alternation.

The pH curve of the probe is of the V_(GS)=C₂·pH+E⁰ type, C₂ being theslope and E⁰ being the intercept point. The values of E⁰ and C₂ varyover time owing to the ageing of the probe.

The in situ calibration is aimed at correcting the intercept pointand/or the slope to make sure that the value of pH measured by means ofthe probe truly reflects reality.

During a calibration cycle, the microcontroller is designed to:

act on the means for generating current to generate a current of knownintensity I₁, at the anode and the cathode for an adjustable duration T₁varying from 1 minute to 30 minutes in order to carry the pH within themembrane to a known value pH₁;

act, at the end of the duration T₁, on the means for generating currentand voltage to generate a constant electric current I_(DS) and aconstant electric voltage V_(DS) between the source and the drain inorder to bias the transistor; activate a measurement in the voltageV_(GS1) between the gate and the source; compute the value of interceptpoint E⁰ in applying the formula E⁰=V_(GS1)−C₂pH₁;

replace the present value of E₀ by the newly computed value.

In this embodiment, the original value of the constant C₂ is kept.

In another embodiment of calibration, the microcontroller is designedto:

act on the means for generating current to generate a first current of aknown intensity I₁, at the anode and the cathode for an adjustableduration T₁ varying from 1 minute to 30 minutes in order to take the pHwithin the membrane to a first known value pH₁;

act, at the end of the duration T₁, on the means for generating currentand voltage to generate a constant electric current I_(DS) and aconstant electric voltage V_(DS) between the source and the drain inorder to bias the transistor;

activate a measurement of the voltage V_(GS1) between the gate and thesource;

memorize the values of V_(GS1), pH₁ and I₁;

act on the means for generating current to generate a second current ofa known intensity I₂, at the anode and the cathode for an adjustableduration T₂ varying from 1 minute to 30 minutes in order to take the pHwithin the membrane to a known value pH₂;

act, at the end of the duration T₂, on the means for generating currentand voltage to generate a constant electric current I_(DS) and aconstant electric voltage V_(DS) between the source and the drain inorder to bias the transistor;

activate a measurement of the voltage V_(GS2) between the gate and thesource;

memorize the values of V_(GS2), pH₂ and I₂;

compute the values of the slope C₂ and the intercept point E⁰ and inapplying the formulae:

C ₂=(pH₁−pH₂)/(V _(GS1) −V _(GS2))

E ⁰ =V _(GS1)−(pH₁−pH₂)/(V _(GS1) −V _(GS2))·ph₁

replace the present values of C₂ and E₀ by the newly computed values.

7.2.2. Operation

A. Measurement of pH

In order to measure the value of the pH of an effluent E, this effluentis put into contact with the reference electrode 26 and with themembrane 29 (hence with the gate and the anode) and with the cathode.

The microcontroller drives the probe so that no current is delivered atthe anode and the cathode.

The H⁺ ions contained by the effluent E then spread inside the membrane29 so that a equilibrium of concentration is obtained between theinterior and the exterior of the membrane 29, i.e. the effluent, forwhich the measurement is made. The concentration in H⁺ ions in themembrane 29 is therefore identical to that of the effluent E.

The microcontroller acts on the means for generating electric currentand voltage to generate a constant voltage V_(DS) as well as acirculation of a constant electric current I_(DS) between the source 21and the drain 22. The values of this electric current and this electricvoltage will be chosen so that they enable the transistor to be biased.

The generation of a voltage is then observed between the source 21 andthe reference electrode 26. This voltage is the voltage V_(GS) of theIFSET transistor between the gate and the source. The value of thisvoltage V_(GS) is proportional to the value of the pH of the effluent E.This voltage varies linearly as a function of the pH for a constantcurrent I_(DS) and a constant voltage V_(DS). The microcontrollerrecords this voltage V_(GS).

The microcontroller then determines the value of the pH of the effluentE according to the value of the electric voltage V_(GS) measured at theterminals of the source 21 and the gate 24 in applying for example theformula:

V _(GS) =C ₂·pH+E ⁰.

B. Calibration

In order to prevent the appearance of a drift between the value of pHmeasured by means of the probe and the real value of the pH of theeffluent, phases of calibration or recalibration of the probe areimplemented regularly, preferably daily.

According to a first embodiment, during the calibration cycle, themicrocontroller acts on the means for generating current to generate acurrent of known intensity I₁ at the anode and the cathode for anadjustable duration T₁ varying from 1 minute to 30 minutes depending onthe time needed to put the concentration in H⁺ ions in equilibrium inthe membrane. This duration is parametrized in the factory. This currentintensity I₁ generates the production of protons for the duration T₁ andthus carries the pH in the membrane to a known value pH₁. The H⁺ ionsgenerated are far greater in quantity than the protons present in theeffluent (in a log relationship), which makes it possible to overlookthe influence of the pH of the water outside the membrane for thecalibration.

At the end of the duration T₁, the microcontroller acts on the means forgenerating current and voltage to generate a constant electric currentI_(DS) and a constant voltage V_(DS) between the source and the drain:the transistor is then biased.

The microcontroller then activates a measurement of the voltage V_(GS1)between the gate and the source. Then it computes the value of theintercept point E⁰ in applying the formula E⁰=V_(GS1)−C₂pH₁. It thenreplaces the present value of E₀ by the newly computed value in theformula V_(GS)=C₂·pH+E⁰.

In another embodiment of calibration, the microcontroller acts on themeans for generating current to generate a first current of a knownintensity I₁ at the anode and the cathode for an adjustable duration T₁varying from 1 minute to 30 minutes depending on the time needed forobtaining equilibrium of concentration of H⁺ ions in the membrane. Thisduration is parameterized in the factory. This current intensity I₁generates the production of protons for the duration T₁ and thus takesthe pH in the membrane to a known value pH₁.

At the end of the duration T₁, the microcontroller acts on the means forgenerating current and voltage to generate a constant electric currentI_(DS) and a constant electric voltage V_(DS) between the source and thedrain: the transistor is then biased.

The microcontroller then activates a measurement of the voltage V_(GS1)between the gate and the source, and memorizes the values of V_(GS1),pH₁ and I₁.

The microcontroller then again acts on the current generating means togenerate a second current of a known intensity I₂ at the anode and thecathode for an adjustable duration T₂ varying from 1 minute to 30minutes to take the pH within the membrane to a second known value pH₂.

At the end of the duration T₂, the microcontroller again acts on themeans for generating current and voltage to generate a constant electriccurrent I_(DS) and a constant electric voltage V_(DS) between the sourceand the drain in order to bias the transistor.

The microcontroller then activates a measurement of the voltage V_(GS2)between the gate and the source, and memorizes the values of V_(GS2),pH₂ and I₂.

The microcontroller then computes the values of the slope C₂ and of theintercept point E⁰ in applying the formulae:

C ₂=(pH₁−pH₂)/(V _(GS1) −V _(GS2))

E ⁰ =V _(GS1)−(pH₁−pH₂)/(V _(GS1) −V _(GS2))·pH₁

It finally replaces the present values of C₂ and E⁰ by the newlycomputed values in the formula V_(GS)=C₂·pH+E⁰.

7.3. Trials

Trials were performed to verify the efficiency of a probe according tothe invention.

These trials consisted in measuring the value of the pH of an effluentwith a classic glass electrode probe and then with a probe according tothe invention.

FIGS. 3 and 4 illustrate:

the real variation of the pH of an effluent: theoretical value;

the variation of the pH value measured by means of a probe according tothe prior art: standard measurements;

the variation of the pH value measured by means of a probe according tothe invention: measurements according to the invention.

As can be seen in these figures, in conducting a calibration step everysix hours, the drift between the real value of the pH and the value ofthe pH measured by means of a probe according to the invention is almostzero or at least appreciably smaller than the drift observed between thereal value of the pH and the value of the pH measured by a prior-artprobe.

7.4. Variant

In one variant, it can be that the membrane will not be implemented. Inthis case, the generation of the current at the anode and the cathodewill modify the value of the pH of the effluent in a controlled mannerin proximity to the gate. In other respects, the structure and theoperation of the probe according to this variant are identical to thoseof the probe comprising the membrane.

In one variant, the reference electrode of the probe could be replacedby a MOSFET (Metal/Oxide/Semi-conductor Field Effect Transistor) typetransistor. In this case, the gate of the MOSFET is electricallyconnected to the gate of the ISFET and a constant current and voltageare applied between the source and the drain of the MOSFET to bias it.The measurement of the voltage at the terminals of the gate of theMOSFET and that of the ISFET which is proportional to the pH of thesolution to be analyzed makes it possible to deduce the pH from this.

In other variants, the reference electrode could be constituted by areference pseudo-electrode made of silver-silver chloride wire, goldwire or other types of wire.

FIG. 5 illustrates an example of an electronic connection diagram of theMOSFET and ISFET transistors of a probe according to this variant.

In this example, a differential cascade is implemented. This cascadecontains a transistor T5 in a feedback connection. The transistors T3and T4 are the active load of the MOSFET and of the ISFET which ensuresthe equality of the drain in these two transistors. The transistor T5(identical to T3 and T4) drives the gate voltage of the MOSFET. The typeof conductivity of the channel of the transistors T3, T4 and T5 isopposite that of the ISFET and the MOSFET. To ensure the equality of thedrain-source voltages V_(DS) of the MOSFET and the ISFET, the conditionI_(CS2)=0.5·I_(CS1) is needed.

Thus, if the potential of the gate V_(G1) of the ISFET increases, thepotential on the source V_(S1) must increase since the drain-sourcecurrent I_(DS1) and the potential at the drain V_(D1) are fixed by allthe transistors T3, T4, T5. Identically, the increase in potential atthe source (V_(S1)=V_(S2)) will lead to an increase in potential at thegate V_(G2) of the MOSFET and therefore of the output voltage V_(out).The potential is measured relatively to the ground of the circuit. Itcorresponds to the voltage between the gates of the MOSFET and the ISFETwhich is proportional to the pH of the solution.

In this embodiment, only the anode and the gate of the ISFET are putinto contact with the effluent to be analyzed.

1-15. (canceled)
 16. Device for measuring the pH of an effluent, saiddevice comprising means for measuring a piece of informationrepresenting the pH of said effluent that are to be put into contactwith said effluent, and means for modifying the value of the pH of saideffluent in proximity to said means for measuring, characterized in thatit comprises means for calibrating said device for measuring, said meansfor calibrating being configured to calibrate the device for measuringafter modification of said pH of said effluent in proximity to saidmeans for measuring by said means for modifying.
 17. Device according toclaim 16, characterized in that said means for measuring comprise: anISFET type transistor comprising a source and a drain disposed on asubstrate, and a gate that is to be put into contact with said effluent;a reference electrode; means for generating a constant voltage at theterminals of said source and said drain; second means for generating aconstant electric current between said source and said drain; means formeasuring a control voltage V_(GS) at the terminals of said source andsaid reference electrode; means for determining the value of the pH ofsaid effluent as a function of the value of said control voltage V_(GS),the value of the control voltage V_(GS) and the value of the pH beingrelated by a formula of the type:V _(GS) =C ₂·pH+E ⁰, where E⁰ and C₂ are predefined constants; and inthat said means for calibrating are configured to implement steps ofcalibration during which: they act on said means for modifying the valueof the pH to take it momentarily to at least one first known value pH₁,then they act on said means for measuring a control voltage V_(GS), tomeasure its corresponding value V_(GS1); they compute the value of theconstant E⁰ according to the values of pH₁ and of V_(GS1).
 18. Deviceaccording to claim 17, characterized in that said means for calibratingare configured to implement steps of calibration during which: they acton said means for modifying the value of the pH to take it momentarilyto a first known value pH₁, then they act on said means for measuring acontrol voltage V_(GS), to measure the value V_(GS1) corresponding tothe first value pH₁; they act on said means for modifying the value ofthe pH to take it momentarily to a second known value pH₂; then they acton said means for measuring a control voltage V_(GS), to measure thevalue V_(GS2) corresponding to the second value pH₂; they compute thevalue of said constants E⁰, C₂ according to the known values of pH₁,pH₂, V_(GS1) and V_(GS2).
 19. Device according to claim 16,characterized in that said means for modifying the value of the pHcomprise an anode and a cathode to be put into contact with saideffluent, and first means for generating electric current between saidanode and said cathode.
 20. Device according to claim 19, characterizedin that said means for modifying the pH comprise command means toimplement or not implement said first means for generating an electriccurrent
 21. Device according to claim 20, characterized in that saidmeans for calibrating are configured to act on said command means toimplement said first means for generating an electric current to modifythe value of the pH during said steps of calibration.
 22. Deviceaccording to claim 19, characterized in that it comprises a membranepermeable to the H⁺ ions covering and being in contact with said anodeand at least partly said means for measuring.
 23. Device according toclaim 22, characterized in that said membrane covers said gate and saidanode, said gate and said anode being in contact with said membrane. 24.Device according to claim 22, characterized in that said membrane ismade of polymer.
 25. Device according to claim 24, characterized in thatsaid polymer is poly(2-hydroxyethylmethacrylate).
 26. Method formeasuring the pH of an effluent by means of a device according to claim16, characterized in that it comprises: a step for measuring the pH withsaid means for measuring; a step for calibrating comprising at least onestep for modifying said pH of said effluent in proximity to said meansfor measuring by said means for modifying, and a step for calibratingsaid device for measuring by said means for calibrating.
 27. Methodaccording to claim 26, characterized in that said step for measuring thepH comprises: a step for generating a constant voltage at the terminalsof said source and said drain; a step for generating a constant electriccurrent at the terminals of said source and said drain; a step formeasuring a control voltage at the terminals of said source and saidreference electrode; a step for determining the value of said pH as afunction of the value of the control voltage V_(GS), the value of thecontrol voltage V_(GS) and the value of the pH being related by aformula of the type: V_(GS)=C₂·pH+E⁰, where E⁰ and C₂ are predefinedconstants; and in that said step for calibrating comprises: a step formodifying said pH by said means for modifying the value of the pH totake it momentarily to a first known value pH₁, then a step formeasuring the corresponding control voltage V_(GS1) by said means (31)for measuring a control voltage; a step of computation by said means forcalibrating of said constant E⁰ as a function of the known values pH₁and V_(GS1).
 28. Method according to claim 27, characterized in thatsaid step for calibrating comprises: a step for modifying said pH bysaid means for modifying the value of the pH to take it momentarily to afirst known value pH₁, then a step for measuring the control voltageV_(GS1) by said means for measuring a control voltage V_(GS), to measurethe value V_(GS1) corresponding to the first value pH₁, then a step formodifying said pH by said means for modifying the value of the pH totake it momentarily to a second known value pH₂, a step for measuringthe control voltage V_(GS2) by said means for measuring a controlvoltage V_(GS) to measure the value V_(GS2) corresponding to the secondvalue pH₂; a step of computation by said means for calibrating of thevalue of said constants E⁰, C₂ as a function of the known values of pH₁,pH₂, V_(GS1) and of V_(GS2)
 29. Method according to claim 27,characterized in that said step or steps for modifying said pH comprisea step for implementing said first means for generating an electricalcurrent.
 30. Method according to claim 26, characterized in that saidstep for calibrating is implemented at a predetermined frequency.
 31. Amethod of measuring the pH of an effluent with a pH measuring device andperiodically calibrating the pH measuring device comprising: engagingthe effluent with a probe that forms a part of the pH measuring device;utilizing the probe to measure a parameter that is representative of thepH of the effluent and determining the pH of the effluent as a functionof the parameter; and periodically calibrating the pH measuring deviceby: (i) adjusting the pH of the effluent in the vicinity of the probe toa known pH; (ii) adjusting the function such that the parameter measuredby the probe yields a measured pH that equals the known pH.